What is this? and how to deal with this?
Well, this occurs when you have faulty feature in the SharePoint environment.

Here are the best tools/techniques in my experience to solve this issue:

Preupgradecheck:This will be our primary analysis and reporting tool providing us with invaluable information regarding your SharePoint 2007 farm and actions that may be required to be taken prior to upgrading to SharePoint 2010. It is good to have October CU:http://support.microsoft.com/kb/976730/ where the preupgradecheck is enhanced further.

To get list of features information:

stsadm -o enumallwebs -includefeatures > FeaturesReport.txt

Then try to De-activate & Un-install the faulty/missing feature first

STSADM -o deactivatefeature -id <feature-id> -url <site-url> -force

stsadm -o uninstallfeature -id <ID>

Feature Admin Tool:
You can Find and delete faulty feature from the farm using Feature Admin Tool. This is a very handy tool,make sure you test before you remove these faulty features. Removing these features should fix most of the errors on the pre-upgrade check.

WSS Analyze Features
Verifies if the feature definition files for all installed features are present on the file system. Also allows to verify if the features used in site collections an sites are installed on the server.WssAnalyzeFeatures -url http://server:port
This command will verify all features on the server and in the site collection.http://archive.msdn.microsoft.com/WssAnalyzeFeatures

Wss Remove Feature From Site
Some times, STSADM.EXE -o deactivatefeature may fail. his tool allows to remove the feature from the site or site collection completly. WssRemoveFeatureFromSite -scope (site|sitecollection) -url <url-to-site> -featureid <featureid> -force http://archive.msdn.microsoft.com/WssRemoveFeatureFrom